Tubular electric resistors and rheostats



Feb. 7, 1961 E. ABRAHAMSON TUBULAR ELECTRIC RESISTORS AND RHEOSTATSFiled June 29, 1959 FIG.

FIG. 3

24 *J I M FIG 43 FIG. 9

/ EDMUN D ABRAHAMSON 5 FIG. 7

k PATENT AGENT tats TUBULAR ELECTREC RESETQRQ AND RHEGSTATS Filed June29, 195?, Ser. No. 823,582

8 Claims. (Ci. 533-27*9} The present invention relates to electricalresistances, such as resistors and rheostats and, more particularly, totubular resistors and rheostats comprising an insulating tube, on whichthe actual electric resistance means ishelically wound in form of a wireor ribbon. The resistance values of these resistors or rheostats can beadjusted or set by means or" a slidable brush or an adjustable endband.

In the operation of such electrical resistors or rheostats in whichheretofore the insulating tubes have had substantially uniform crosssections over their lengths, it has been found that the turns of theresistance wires tend to burn out close to or toward one end of thewinding because either the movable slide brush or endband is frequentlyset in these resistors or rheostats in such a manner, that only a fewturns of the winding are inserted in the electric circuit, while nocurrent will flow through the remaining turns. As a result of this kindof operation, the few resistance wire turns in the circuit arerelatively highly heated, because the total resistance of the circuit islow when the larger part of the resistors or rheostats is not insertedin the circuit and the amount of heat generated in these few resistanceturns is a function of the square of the current intensity times theresistance value inserted in the circuit.

It is an object of the present invention to improve the thermalcondition or heat dissipation in a resistor or rheostat of the tubulartype, i.e., to distribute the heat developed in the end turns of suchresistance more evenly throughout the entire apparatus when on y theseend resistance turns are inserted in the electric circuit.

It is another object of this invention to provide as supportinginsulating tube for the turns or winding of the resistance wire a tubehaving an inner diameter which is larger at that end of the tube wherethe turns of the resistance wire are Wound, said turns being insertedeither permanently or for a longer period of the operation in thecircuit to be controlled by the resistance.

It is a further object of the invention to provide an insulating tubewith a tapering inner cross section, whereby the turns of the wirewinding remaining permanently or for a longer period in the circuit areat the end with the smaller internal opening of the tube.

It is a sti l further object of the invention to provide a tube assupport for the wire winding of the resistor or rheostat, said tubehaving at least two different inner diameters, i. e., comprisingsections with at least two diiferent diameters, of which the smallestone is at the end carrying the turns of the wire winding remainingpermanently or for a longer period of the operation in the electriccircuit.

It is another object of the invention to divide the insulating tubesalong their lengths into two parts which can be manufactured moreeconomically and then put them together prior to the winding of the wireon the tube.

Still further objects and the entire scope of applicabi ity of thepresent invention will become apparent from the detailed descriptiongiven hereinafter; it should be understood, however, that the detaileddescription and specific atent examples, while indicating preferredembodiments of the invention, are given by way of illustration only,since various changes and modifications within the spirit and scope ofthe invention will become apparent to those skilled in the art from thisdetailed description.

in the drawings:

Figure 1 shows schematically a circuit including a rheostat according tothe present invention;

Figure 2 is a longitudinal section through an insulating tube accordingto the present invention taken along the line 2-2 of Figure 3 andlooking in the direction of the arrows;

Figure 3 is a top view of the tube of Figure 2;

Figure 4 shows schematically a circuit including a potentiometer in formof a rheostat or resistor according to the present invention;

Figure 5 shows a top view of a modified insulating tube which is dividedinto two halves along its length;

Fi ure 6 shows a perspective view of the tube of Fi ure 5 with endbandsassembled on their ends ready to re wound with resistance wire;

Figure 7 shows a top view of a modified insulating u e similar to thatshown in Figures 5 and 6;

i are 8 shows a longitudinal section through another modified tubeaccording to the invention;

i "gure 9 illustrates a longitudinal section through an insulating tubeaccording to the present invention, said tube being a modification ofthat of Figure 8.

Referring in detail to the drawings, Figure 1 shows a rheostat itinserted in an electric circuit comprising a battery 12 and a load 12.The rheostat 1.0 has a helical resistance winding 13 supported on aninsulating tube 14, for example of porcelain. While one terminal of theload 12 is connected to the right end of the resistance wire, the oneterminal of the battery 11 is connected to a slide arm or brush 15frictionally engaging the one or the other turn of the resistance wire13 on the tube 14 of the rheostat ill and the other terminal of thebattery H is connected to the other terminal of the load 12, whereby thedesired current intensity value in the load circuit can be adjusted bychanging the position of the slide brush 15 along the tube 14. Assumingthat during operation this slide brush 15 remains closer to the rightend of the rheostat 1%, i.e., that only a part of the rheostat isinserted in the circuit, as it is very often true in such load circuits,then the resistance wire turns on the right end of the rheostat 10 willbe subjected to a greater wear than the remaining turns of the rheostat,due to the unfavorable heat dissipation and distribution in therheostat. According to this invention, the insulating tube on which theresistance wire is wound is conically tapering, i.e., the interior crosssection is increasing towards the end of greater thermal stress, so thatthe heat produced in the turns of the resistance wire can be betterdissipated by the insulating tube 14, i.e., better absorbed by thelarger cross section of the same at the right portion thereof.

Figures 2 and 3 show the actual design of such rheostat tube it havingat the end of the smallest inner diameter, i.e., the largest crosssection, a flare 16 necessary or desirable in view of the brittleness ofceramic material, such as porcelain, mostly used for such tubes.

Fi ure 4 shows a similar circuit as that illustrated in Figure 1. Inthis case, a rheostat 2G is inserted in a circuit and used aspotentiometer or voltage divider, the helical resistance winding 21 ofwhich with its two ends 22. and 23 being permanently connected to theterminals of a battery 24. Potentiometer taps 25 and 26 along theresistance winding 21 are connected to a low current load 27 and a highcurrent load 28, respectively, the other terminals of said loads beingconnected to one of the battery terminals. This kind of potentiometerused for tapping ofi currents of controlled intensities has been knownper se.

As shown in Figure4, the insulating tube 14 is provided as in Figure 1with an interiorly tapering cross section to improve the heatdissipation, i.e., to provide the largest cross section of the tube atthe place of highest heat development, i.e., on the right side thereof.

In view of difliculties in the manufacture of integral tubes havingvarying. cross sections over their lengths, these tubes are made,according to the invention, of two equal parts or halves 30 and 31, asshown in Figures and 6, with the dividing lines along the length ofthese tubes. Each of the engaging surfaces of the halves 3t) and 31 ofthese tubes has a projection 32 on one side and a notch or recess 33 onthe other side, whereby opposite projections and notches engage oneanother, so that the tubes after being assembled are securely heldtogether when clamped by endbands 34 for the resistance wire at the endsof the tubes, as shown in Figure 6. The resistance wire a (not shown) isthen wound on the tube thus assembled and clamped together by theendbands 34, as shown in Figure 6. As the two halves 30 and 31 areidentical in-shape, they can be formed or molded in the same mold. Inother words, only a single half-shell mold is necessary for themanufacture of the two halves.

Figure 7 shows a'somewhat different configuration of the engagingsurfaces of the parts or halves 36 and 37 of the tube, said parts orhalves fitting together. In this embodiment, the interengaging surfacesof the tube halves 36 and 37 form steps 38.

Instead of providing a tapering cross section, it is possible to designthe tubes according to the invention with different cross sections alongtheir lengths, for example, as shown in Figure 8. In this case, theright end 39 has a larger cross section than the remaining portion 40 ofthe tube. The turns of the resistance winding more frequently used willbe located on the right end 39 of the tube, as shown in Figure 8. Thus,the heat produced in the resistance wire turns at this end will be morereadily dissipated.

According to the embodiment of Figure 9, the insulating tube has threedifferent cross sections starting from a largest 41 on the right side toa smallest 43 on the left side with an intermediate 42 therebetween.Such tube will be advantageously used in case where the turns of theresistance wire mostly used in the circuit are extending from the rightend to the center of the tube.

The heat distribution will be further improved by mounting the resistoror rheostat with the porcelain tube substantially arranged in verticalposition, as shown in Figure 2, whereby the end of the tube with thelarger cross section is facing downwardly. As a result of this, heat isremoved by the air current flowing through the interior of the tube in asuperior manner, due to the chimney efiect.

I claim:

1. An electric resistance comprising an insulating tubular supportingmember, a helical winding of resistance wire on said tubular supportingmember, means to adjustably connect the turns of said resistance wire toan electric circuit, the external diameter of said tubular memberremaining substantially constant throughout the length of said memberand the internal diameter of said tubular member varying in crosssection throughout the length of said tubular member to impart to saidtubular members a greater wall thickness at one end than at the other.

2. An electric resistance according to claim 1, wherein said tubularmember is internally tapered towards said one end.

3. An electric resistance according to claim 1, wherein said insulatingtubular member has at least two different internal diameters definingsections each of which has substantially the same wall thicknessthroughout its respective length.

4. An electric resistance according to claim 1, wherein said insulatingtubular member is divided along its length in two halves fittingtogether.

5. An electric resistance according to claim 4, wherein the engagingsurfaces of said two halves are respectively provided with projectionsand recesses interengaging one another.

6. An electric resistance according to claim 5, wherein one of saidsurfaces of each half has a projection and the other surface has arecess.

7. An electric resistance according to claim 4, wherein steps are formedon the engaging surfaces of said two halves, said steps respectivelyinterengaging one another.

8. In an electric load circuit including a source of current and a load,an electric resistance comprising a tubular supporting member, a helicalwinding of resistance wire on said members, means to adjustably connectthe turns of said resistance wire into said load circuit,

the external diameter of said tubular member remaining substantiallyconstant throughout the length of said member and the internal diameterof said tubular supporting member varying in cross section throughoutthe length of said tubular member to impart to said tubular member agreater wall thickness at one end than at the other, the portion of saidtubular member with the greatest wall thickness supporting theresistance wire turns which are inserted in said load circuit during thelongest period of operation.

References Cited in the file of this patent UNITED STATES PATENTS

